Mechanical jacks are widely used to lift heavy loads, usually up to 8,000 lbs. (3,620 kg). A typical mechanical jack comprises a vertical support slidably supporting an upper lifting assembly and a lower follower assembly, the lifting and follower assemblies being engagable to the support at a series of predetermined latching positions along its length. An operating handle has one end pivotally connected to the lifting assembly and a link pivotally connected between the handle and the follower assembly. The lifting assembly includes a reversing member which is set in one position for lifting a load and in another position for lowering a load.
With the reversing member set to the lifting position, a load can be raised by reciprocation of the handle, which in the down-stroke moves the lifting assembly to the next higher latching position and in the up-stroke moves the follower assembly to the next higher latching position. With the reversing member set to the lowering position, the load can be lowered by controlled resistance applied to the handle as the load forces the handle upwardly, the lifting assembly latching into the next lower latching position as the handle nears the top of its stroke. An example of such a jack is described and illustrated in U.S. Pat. No. 4,568,064 issued Feb. 4, 1986 to Reinhardt, which is incorporated herein by reference.
Mechanical lifting jacks can be extremely dangerous if used improperly. When lowering a load the upward force on the handle can be sufficient to cause the operator to lose his or her grip. If the operator's hands slip off of the handle before the lifting assembly has latched into the next lower latching position on the vertical support, the handle can swing upward violently with the potential to cause serious injury to the operator.
The present invention overcomes this problem by providing a mechanical lifting jack with a locking mechanism that arrests upward motion of the handle if the handle is suddenly released by the operator while lowering a load. The invention accomplishes this by providing a latch on the pivot attachment of the handle to the lifting assembly, actuated by a lever disposed at the distal end of the handle, and a toothed safety link into which the latch engages whenever the lever is released.
Thus, in the event that the operator suddenly releases the handle accidentally before the lifting mechanism has locked into the next lower latching position on the vertical support, the latch engages the safety link to arrest the upward motion of the handle. The invention can also be used to lock the handle into an upright position alongside the vertical support, for compact storage.
The present invention thus provides a mechanical lifting jack having a vertical support provided with a series of apertures, an upper lifting assembly and a lower follower assembly having a lifting foot slidably mounted on the vertical support and engagable therewith at predetermined positions, an operating handle pivotally connected by a yoke to the lifting assembly for vertical movement, a safety link pivotally connected at an upper end to the handle and at a lower end to the follower assembly, and a reversing member pivotable between a lifting position for lifting the load by reciprocal movement of the handle causing upward movement alternately of the lifting and lowering assemblies and a lowering position for lowering the load by reciprocal movement of the handle causing downward movement alternately of the lifting and lowering assemblies, a latching member extending between opposed arms of the yoke, movable between a latching position and a retracted position and biased to the latching position, a lever projecting from the handle operatively connected to the latching member, to move the latching member between the latching position and the retracted position, and a rotationally fixed toothed element extending into a space between the arms of the yoke, whereby when the latching member is engaged into the toothed element the handle is prevented from substantial upward or downward movement and depression of the lever retracts the latching member from the toothed element and allows the handle to move through its normal operating motion.
The present invention further provides a safety latch for a mechanical lifting jack having a vertical support provided with a series of apertures, an upper lifting assembly and a lower follower assembly having a lifting foot slidably mounted on the vertical support and engagable therewith at predetermined positions, an operating handle pivotally connected by a yoke to the lifting assembly for vertical movement, a safety link pivotally connected at an upper end to the handle and at a lower end to the follower assembly, and a reversing member pivotable between a lifting position for lifting the load by reciprocal movement of the handle causing upward movement alternately of the lifting and lowering assemblies and a lowering position for lowering the load by reciprocal movement of the handle causing downward movement alternately of the lifting and lowering assemblies, the safety latch comprising a latching member extending between opposed arms of the yoke, movable between a latching position and a retracted position and biased to the latching position, a lever projecting from the handle operatively connected to the latching member, to move the latching member between the latching position and the retracted position, and a rotationally fixed toothed element extending into a space between the arms of the yoke, whereby when the latching member is engaged into the toothed element the handle is prevented from substantial upward or downward movement and depression of the lever retracts the latching member from the toothed element and allows the handle to move through its normal operating motion.